1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor laser device, and more particularly to a method for manufacturing a semiconductor laser device, which prevents various failures caused by protrusions formed on the upper surface of a current blocking layer and an amorphous and/or polycrystalline layer formed at a partial area of the protrusions.
2. Description of the Related Art
Recently, semiconductor laser diodes have been adapted as light sources for light pick-up devices of optical disk systems such as CDs or DVDs in the optical communication field, and have been further applied in various fields such as multiplex and space communications. The reason is that a laser beam generated from a semiconductor laser diode has a narrow frequency bandwidth (short wavelength property) and high directivity, and assures high output power.
Generally, in order to allow current injection efficiency of a semiconductor laser device to be improved, the semiconductor laser device employs a p-type clad layer with a selectively buried ridge (SBR) structure. FIGS. 1a and 1b show a conventional semiconductor laser device 20.
As shown in FIG. 1a, the semiconductor laser device 20 comprises a substrate 11 provided with a first electrode 21 formed on the lower surface thereof, a first conductive-type clad layer 12 formed on the substrate 11, an active layer 13 with a multi-quantum well structure, a second conductive-type clad layer 14 provided with a ridge structure, and a cap layer 15 formed on the upper surface of the ridge structure.
The semiconductor laser device 20 further comprises a current blocking layer 18, made of a first conductive-type material, formed on the upper surface of the second conductive-type clad layer 20 around the ridge structure, and a contact layer 19 and a second electrode 22 sequentially formed on the cap layer 15 and the current blocking layer 18. A buffer layer (not shown) suitable for the lattice matching may be additionally interposed between partial layers (for example, between the substrate 11 and the first conductive-type clad layer 12) of the obtained crystalline structure.
More specifically, in a semiconductor laser device for generating a laser beam with a wavelength of 650 nm, which is adapted to reproduce a DVD, an n-type GaAs substrate is used as the substrate 11, and an n-type AlGaInP layer and a p-type AlGaInP layer are used as the first conductive-type clad layer 12 and the second conductive-type clad layer 14, respectively. Further, the active layer 13 has the multi-quantum well structure for generating light with a wavelength of 650 nm, and a p-type GaAs layer is used as the cap layer 15.
A mask (not shown) made of a material such as SiO2 is located at a current injection area of the cap layer 15, and then a wet-etching process is performed on the cap layer 15 and the second conductive-type clad layer 14, thereby forming the ridge structure shown in FIG. 1a. In order to prevent the active layer 13 from being damaged during the wet-etching process, an etching-blocking layer (not shown) may be additionally provided at a designated depth of the second conductive-type clad layer 14.
After the mask is removed from the current injection area of the cap layer 15, the current blocking layer 18 is formed. An n-type GaAs layer, which is doped with an impurity having a conductive type different from the second conductive-type clad layer 14. Then, the p-type GaAs contact layer 19 and the second electrode 22 are sequentially formed thereon. Thereby, the semiconductor laser diode 20 shown in FIG. 1a is obtained.
Since the current blocking layer 18 is formed along the side wall of the ridge structure of the second conductive-type clad layer 14, the current blocking layer 18 has protrusions A inclined at a designated angle. Particularly, the protrusions A of the current blocking layer 18 have a relatively large size such that they are formed over almost the entire area of the current blocking layer 18, and an amorphous or polycrystalline layer is formed on a dielectric mask and an interface between the dielectric mask and the current blocking layer 18.
Accordingly, the protrusions A of the current blocking layer 18 and the amorphous or polycrystalline layer formed at a partial area of the current blocking layer 18 may have an undesired affect on the crystal growth of the p-type GaAs contact layer 19 formed thereon. Thereby, V-shaped grooves B are formed in the upper surface of the p-type GaAs contact layer 19.
FIG. 1b is a photograph of a cross-section of a semiconductor laser device manufactured by the conventional method. As shown in FIG. 1b, the amorphous or polycrystalline layer formed at the partial area of the current blocking layer 18 and the protrusions A cause a poor surface state of the p-type GaAs contact layer 19 formed thereon and an uneven surface of the electrode formed thereon, thus causing a failure in the connection between the electrode and the device and more particularly electrode cutting.
As described above, the surface state of the current blocking layer 18 and the V-shaped grooves B formed by the amorphous or polycrystalline layer formed thereon may cause severe failures during the following step. For example, a metal material is formed over the entire surface of the device during an electrode forming step for an Ohmic contact, thereby forming an undesired electrode, and cracks are formed in the upper surface of a chip during a chip cleaving step.